Field of the Invention
The present invention relates to solid state laser modules and laser systems that incorporate solid state laser modules.
Description of Related Art
Laser devices that incorporate solid state laser modules are used for a variety of purposes including but not limited to applications such as target marking, pointing, designating, aiming, data communication, and stand-off spectral analysis. These applications require laser systems that are capable of directing laser beam through free space to create a laser spot on a distant target that reflects enough light to be observed or sensed by an electronic sensor such as a spectrometer, a spectrophotometer or an array type image sensor that may itself be a great distance from the target. To accomplish this, it is necessary to provide a laser beam that forms a spot of laser light on the distant target having at least a minimum intensity. Such applications require a laser system that can emit a laser beam with limited divergence.
In many of these applications, it is also necessary that such laser devices are in a form that is easily transported to a point of use and that can be readily and reliably operated when needed. This requires laser devices that are lighter, have a smaller cross-sectional area and reduced volume while still being robust enough to operate after being exposed to vibration, tension, compression, bending forces, torsional loads, and environmental extremes during transport and use. It is particularly important in military, homeland security, first responder applications that such laser devices will remain operational even when exposed to high levels of shock.
However, it is challenging to design and assemble smaller laser devices. In part this is because the natural inclination of engineers and scientists is to achieve size reduction through the simple expedient of downscaling extent designs and processes. However, in the field of laser technology simply using smaller versions of components to achieve system size reductions rarely achieves reliable results.
An example of the complications that can arise when seeking to downsize laser systems through downscaling components can be illustrated with reference to U.S. Pat. No. 7,492,806, entitled “Compact Mid-IR Laser” which describes what is known in the art as a high heat load laser module.
FIG. 1 shows an exploded isometric view adapted from FIG. 2A of the '806 patent. FIG. 2 shows generally a front elevation view of such a high heat load laser module. As can be seen in FIG. 1, high heat load laser module 10 has a housing 12 that can be on the order of 3 cm×4 cm×6 cm. Housing 12 contains a quantum cascade laser 14 that emits infrared light, a submount 16 on which quantum cascade laser 14 sits. A heat spreader 18 has a keyway 20 within which submount 16 is mounted. Housing 12 also contains drive circuitry 24, a thermoelectric cooling system 26, a lens 28 and conductors 30 that extend through sidewalls of sealed box 12 to supply electrical energy and control signals to drive circuitry 24, a thermoelectric cooling system 26 and quantum cascade laser 14.
Box 12 is comprises a front wall 32 with an opening 34 and a window 36, a lid 38, a base 40 and a rear wall 48. Heat spreader 18 is positioned so that quantum cascade laser 14 directs divergent infrared light toward window 36. Lens 28 is positioned between quantum cascade laser 12 and window 36 to collect infrared light emitted by quantum cascade laser 14 and to focus this light.
The '806 patent uses a small focal length lens and asserts that “the small focal length of the lens is important in order to realize a small overall footprint of the laser device.” The lens 28 “may comprise an aspherical lens with a diameter approximately equal to or less than 10 mm and preferably approximately equal to or less than 5 mm. Thus, the focal length may be one of approximately 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, or 20 mm and any fractional values thereof. The focal length of the lens 14 is fabricated to be approximately ½ the size of the diameter. Thus, 10 mm diameter lens will have a focal length of approximately 5 mm, and a 5 mm diameter lens will have a focal length of approximately 2.5 mm.”
Similarly, the importance of using a small or short focal length lens to provide smaller sized laser systems has been described in other patents including U.S. Pat. No. 7,535,656, filed on Sep. 22, 2006 which notes that “embodiments include optical components arranged as mid-IR, high numerical aperture, short focal length lenses. In particular, lenses which have a high transmission for mid-IR spectra further having numerical apeatures greater than about 0.7 and a focal length less than about 10 millimeters” and later defines the term short focal length lens as referring “to lenses and focusing mirrors which have a focal length less than about 8 millimeters. As there is no certain meaning associated with a lens having f=0, we declare a lower limit on our ‘short focal length’ to be about 0.5 millimeters. Any value between 0.5 and 8 millimeters is herein considered a short focal length.”
However, the design decision to use a small or short focal length lens 28 presents a number of design challenges. First, this requires that lens 28 is in close proximity to quantum cascade laser 14. In the '806 patent, this is accomplished by placing lens 28 and lens support structures 35 inside box 12 adding to the challenges of designing and manufacturing box 12.
This in turn requires positioning lens 28 with a high degree of accuracy relative to quantum cascade laser 14. The '806 patent seeks to meet this requirement by utilizing the heat spreader 18 as an optical platform, noting that “[t]he output lens . . . and laser gain medium . . . are held in a secured, fixed and rigid relationship to one another by virtue of being fixed to the optical platform. The use of the heat spreader . . . as a monolithic support block for both lens . . . and laser gain medium . . . is said to be among “other factors contributing to the small footprint include the monolithic design of the various elements, particularly as related to the positioning of the optical components and the ability to efficiently remove the large amount of heat from the QCL serving as the laser gain medium.
The '806 patent also describes positioning heat spreader 18 on thermoelectric cooling system 26 and the thermoelectric cooling system 26 in turn is positioned on a surface that can transfer heat from box 12. Thermoelectric cooling system 26 is used to ensure that heat spreader 18 maintains a temperature where heat spreader 18 holds the output lens [28] and laser gain medium [14] are held in the secured, fixed and rigid relationship.
This approach requires an increase in the size of thermoelectric cooling system 26, an increase in the thermal mass that must be cooled by thermoelectric cooling system 26 and a concomitant increase in the amount of energy that will be consumed by thermoelectric cooling system 26 during use. This, in turn, reduces the overall efficiency of a laser device that uses high heat load laser module 10, and increases the weight and size of portable power supplies that must be provided in a laser device that incorporates high heat load laser module 10.
Additionally, limitations on the size and position of lens 28 can limit the ability of lens 28 to reduce the divergence of the mid-wave or long-wave infrared light emitted by quantum cascade laser 14. Thus, where it is desirable that a laser device that uses high heat load laser module 10 provides a collimated beam additional focusing optics outside of high heat load laser module 10 are generally needed. This, in turn, requires that a laser device that uses high heat load laser module 10 provide additional optical elements and mounting structures. The additional optical elements and mounting structures further increase the volume, weight, complexity, and cost of the laser device while introducing additional vectors through which the reliability and ruggedness of the laser device may be reduced.
As is also shown in FIGS. 1 and 2, high heat load laser module 10 includes a plurality of electrical conductors 30 that project from sidewalls 42 and 44. As is apparent from FIG. 2, the use of side wall projecting electrical conductors 30 in high heat load laser module 12 causes high heat load laser module 12 to have a significant cross sectional area causing a laser device that incorporates high heat load laser module 12 to have an even larger cross sectional area.
For example, as is shown in FIG. 2, a laser device that is to incorporate high heat load laser module 12 in, for example, a cylindrical package, will have a minimum theoretical radius 48. However, in application a laser device that uses high heat load laser module 12 will be required to provide connectors (not shown) that connect to ends of conductors 30 and therefore will have a larger required radius to accommodate laterally joined connectors. Further, to the extent that it is desired to enclose the connectors and the laser module, such an enclosure will add additional radius and cross sectional area.
Smaller high heat load laser modules are known. For example the DFB-CW type quantum cascade lasers sold by Hamamatsu Corporation, Japan include conductors that project from one or the other of sidewalls of a box, however the size enhancing effects of side projecting conductors are only partially ameliorated by this approach.
Another concern with the high heat load laser module 12 is that high heat load laser modules 12 are highly specialized designs that are intended for specific purposes and that do not easily allow modification and repurposing. Accordingly any use of such a module for another purpose requires adaptive electronics, hardware and optics, which increase the cost, size, weight and complexity of a laser emitting product.
It will be appreciated from this that other approaches are needed.
One alternative approach is shown in German patent publication number DE10205301A1, filed on Feb. 2, 2002. FIG. 3 is a side view of a laser device adapted from the '301 publication. This patent publication describes the use of a laser device 50 having a quantum cascade laser LED 52 that is contained between a base section 54 and a housing section 58. Electrical conductors pass through base 54 and extend along an axis 60 in a first direction, while quantum cascade laser 52 is arranged on a pedestal 56 on base 54 to emit light generally along axis 60 in a second direction.
In one embodiment, housing 58 has a front wall section 62 with an opening 64 through which radiation from quantum cascade laser 54 can pass. A lens 66, preferably a Fresnel lens, is inserted the opening 64. Lens 66 collimates a light emitted by quantum cascade laser 54 to form a beam. As is described in the '301 application, a distance d separates the quantum cascade laser 54 and lens 66. To achieve desired collimation the distance d is equal to a focal length of lens 66.
Another laser module design has been proposed in US Pat. App. No. 2005/0008049 entitled “optical module including a Peltier device therein and having a coaxial type package” filed by Oomori et al. on Jun. 2, 2004.
FIG. 4 shows a cutaway portion isometric view of a co-axial laser module 70 and adapted from the '049 application and FIG. 5 shows an end view of laser module 70. As is shown in FIGS. 4 and 5 conductors 72 are positioned on one side of a base 74 and a semiconductor diode laser 76 is positioned on the opposite side of base 74 and enclosed by a housing 78. An opening 82 is provided in housing 78 to provide a light path out of housing 78 and a lens 84 is positioned in opening 82 to reduce the extent of the divergence of light emitted by semiconductor diode laser 76.
This approach attempts to solve some of the aforementioned problems by providing a thermo-electric cooler 86 between base 74 and housing 78 and assembling a support 88 for a semiconductor diode laser 76 onto thermoelectric cooler 86 to provide a shorter path for heat transfer between semiconductor diode laser 76 and thermoelectric cooler (described as a Peltier device) 86.
However, this approach makes the process of designing a laser module in a coaxial design more challenging by requiring the installation of thermoelectric cooler 84 within the coaxial package, the connection of additional electrical connectors to thermoelectric cooler 86, the mounting of a stem at an angle that is normal to the mounting surface on thermoelectric cooler 86, and the mounting of a semiconductor laser diode to support 88 at an angle that is normal to support 88. Further, this approach limits the overall size and thermal management capabilities of the thermoelectric cooler 84.
Still another approach to providing an alternative to a high heat load package is described in U.S. Pat. No. 8,442,081 entitled “Quantum Cascade Laser Suitable for Portable Applications” filed on Apr. 25, 2012. This describes a laser device with a lens system at a first end that emits collimated infrared light and a cap at a second end. A quantum cascade laser directs divergent laser light toward the lens. A control circuit and a battery storage area are positioned between the quantum cascade laser and the cap. Heat from the quantum cascade laser passes from the quantum cascade laser through a heat spreader providing what is described as a short path to a laser housing to provide a limited amount of passive cooling. A drive circuit operates the quantum cascade laser at a low duty cycle to limit the amount of heat generated by the quantum cascade laser to an amount that can be dissipated by the limited amount of passive cooling.
Effectively the package described in the '081 patent offers a designer a simple tradeoff: forgo the problems associated with the use of a thermoelectric cooling device at the cost of limiting laser use to a level only generates so much heat as can be passively dissipated by the laser system.
Additionally, it will be appreciated that, the use of laser module housings to position lens systems as is shown in the '301 publication, the '049 application and the '081 patent imposes a number of constraints on the design of the laser module. For example, the laser module housing must be designed and assembled so that the lens is held in optical alignment with the quantum cascade laser that is supported on the base and is separated from the quantum cascade laser by the focal distance of the lens. Additionally, the housing and the lens must be designed and assembled such that the relative alignment and positioning of the lens and the quantum cascade laser do not change because of changing thermal conditions that arise during operation laser or changing environmental conditions.
Similarly, positioning a lens at an opening of a laser module housing places many constraints on the optical properties of the lens and the mountings used to join the lens to the housing. For example, cross-sectional area constraints housing, volume constraints of the housing, as well as mechanical and thermal properties of the housing will impose limitations on the size and shape of the lens.
Another challenge that must be met when the lens is placed in the opening of a laser module housing arises when the laser module is to form part of a hermetic or other seal to provide a controlled environment around the quantum cascade laser. For example, in some cases it can be advantageous to operate the quantum cascade laser in a low pressure or vacuum environment or to operate the quantum cascade laser in the presence of inert gasses or even to simply limit the humidity in the environment about the quantum cascade laser. Where it is beneficial to provide such controlled environments within a laser module, a seal, typically a hermetic seal is required and it is necessary that both the lens and lens mounting are designed and assembled to operate as a function of the seal. However, it is also necessary that the optical functions of the lens and the lens mounting are not compromised by incorporating the lens and the mounting into the design of the housing.
This requires new and innovative approaches to providing lens mountings, lens designs and assembly processes that can be efficiently used to establish an effective seal while also allowing the lens to be positioned at a precise optical alignment and relative position to the quantum cascade laser.
It will be understood that these design challenges are not easily met. Accordingly, what is still needed in the art is a new laser module package with reduced cross-sectional area requirements, reduced volume requirements, reduced weight, greater efficiency, reduced complexity, greater ease of manufacturability, increased portability, greater resistance to shock, vibration, tension, compression, and bending, thermal variations, and environmental conditions.
As is noted above, there are many possible applications for solid state lasers. These applications typically have different performance requirements including but not limited to different laser coherence, divergence, output powers, ruggedness and portability requirements, efficiencies, intensities and wavelengths. Accordingly, each different application may have other specialized optical, mechanical or electrical features. When such systems are packaged with lenses that are internal to or otherwise joined to a laser housing there is an inherent linkage between the laser used and the system requiring redesign of the entire module for each compact laser/lens solution.
What are needed therefore, are laser systems and laser modules for use in laser systems that are smaller, lighter more compact and less complex without introducing the reliability, design, and manufacturing complexities.